During the over twenty-year period that melt-blown fibers have come into wide commercial use, for uses such as filtration, battery electrode separation and insulation, there has been a recognized need for fibers of extremely small diameters and webs of good tensile strength. However, there has always been a recognition that the tensile strength of melt-blown fibers was low, e.g., lower than that of fibers prepared in conventional melt-spinning processes (see the article "Melt-Blowing--A One-Step Web Process For New Nonwoven Products," by Robert R. Buntin and Dwight D. Lohkcamp, Volume 56, No. 4, April 1973, Tappi, Page 75, paragraph bridging columns 2 and 3). At least as late as 1981, the art generally doubted "that melt-blown webs, per se, will ever possess the strengths associated with conventional nonwoven webs produced by melt spinning in which fiber attenuation occurs below the polymer melting point bringing about crystalline orientation with resultant high fiber strength" (see the paper "Technical Developments In The Melt-Blowing Process And Its Applications In Absorbent Products" by Dr. W. John McCulloch and Dr. Robert A. VanBrederode presented at Insight '81, copyright Marketing/TechnoLogy Service, Inc., of Kalamazoo, Mich., page 18, under the heading "Strength").
The low strength of melt-blown fibers limited the utility of the fibers, and as a result there have been various attempts to combat this low strength. One such effort is taught in Prentice, U.S. Pat. No. 3,704,198, where a melt-blown web is "fuse-bonded," as by calendering or point-bonding, at least a portion of the web. Although web strength can be improved somewhat by calendering, fiber strength is left unaffected, and overall strength is still less than desired.
Other prior workers have suggested blending high-strength bicomponent fibers into melt-blown fibers prior to collection of the web, or lamination of the melt-blown web to a high strength substrate such as a spunbond web (see U.S. Pat. Nos. 4,041,203, 4,302,495 and 4,196,245). Such steps add costs and dilute the microfiber nature of the web, and are not satisfactory for many purposes.
With regard to fiber diameter, there is a recognized need for fibers of uniformly small diameters and extremely high aspect ratios, as discussed, for example in Hauser U.S. Pat. No. 4,118,531 (col. 5) and Kubik et al. U.S. Pat. No. 4,215,682 (cols. 5 and 6). However, as recognized by Hauser, despite the ability to get melt-blown fibers with very small average fiber diameters, the fiber size distribution is quite large, with fibers in the 6 to 8 micrometer range present for use with fibers of an average fiber diameter of 1 to 2 micrometers (Examples 5-7). Problems are also present in eliminating larger diameter "shot", discussed in the above Buntin et al. article, page 74, first paragraph of col. 2. Shot is formed when the fibers break in the turbulence from the impinging air of the melt-blown process. Buntin indicates that shot is unavoidable and of a diameter greater than that of the fibers.
McAmish et al, U.S. Pat. No. 4,622,259, is directed to melt-blown fibrous webs especially suitable for use as medical fabrics and said to have improved strength. These webs are prepared by introducing secondary air at high velocity at a point near where fiber-forming material is extruded from the melt-blowing die. As seen best in FIG. 2 of the patent, the secondary air is introduced from each side of the stream of melt-blown fibers that leaves the melt-blowing die, the secondary air being introduced on paths generally perpendicular to the stream of fibers. The secondary air merges with the primary air that impacted on the fiber-forming material and formed the fibers, and the secondary air is turned to travel more in a direction parallel to the path of the fibers. The merged primary and secondary air then carries the fibers to a collector. The patent states that, by the use of such secondary air, fibers are formed that are longer than those formed by a conventional melt-blowing process and which exhibit less autogeneous bonding upon fiber collection; with the latter property, the patent states it has been noted that the individual fiber strength is higher. Strength is indicated to be dependent on the degree of molecular orientation, and it is stated (column 9, lines 21-27) that the high velocity secondary air employed in the present process is instrumental in increasing the time and distance over which the fibers are attenuated. The cooling effect of the secondary air enhances the probability that the molecular orientation of the fibers is not excessively relaxed on the deceleration of the fibers as they are collected on the screen. Fabrics are formed from the collected web by embossing the webs or adding a chemical binder to the web, and the fabrics are reported to have higher strengths, e.g., a minimum grab tensile strength-to-weight ratio greater than 0.8 N per gram per square meter, and a minimum Elmendorf tear strength-to-weight ratio greater than 0.04 N per gram per square meter. The fibers are also reported to have a diameter of 7 micrometers or less. However, there is no indication that the process yields fibers of a narrow fiber diameter distribution or fibers with average diameters of less than 2.0 micrometers, substantially continuous fibers or fiber webs substantially free of shot.